Drill stem test tools

ABSTRACT

Once a new oil well has been drilled and cased, a test string is set in place for the purpose of evaluating the production potential of the chosen formation. One way of controlling the operation of the various tools included in the downhole test string, including the opening and closing of the downhole valve itself, is by changes in the pressure differential between the tubing and the annular space which surrounds it in the well, but this requires the provision and maintenance of a fixed &#34;reference&#34; pressure within the tool, and a convenient such pressure is the hydrostatic (annulus) pressure experienced by the string after it has been lowered down the well bore and set into the packer. 
     The invention proposes that reference pressure within the test string be trapped by a novel mechanism wherein a valve (4) drivable into a closed position by a first piston (3) open (at 5) to annulus pressure first defines, and then defines and closes, the open-to-tubing-pressure entrance (6) to a passageway (30/19) leading to a reference-gas-containing chamber (22) via a second piston (20) therewithin. The invention also proposes a new mechanism by which compensation can be made for the effect of downhole temperature changes on the gas in a reference pressure chamber, in which mechanism there is a hydraulic-liquid-containing chamber (27) which is connected at one end, via a piston (25) thereat, to a vent (24) to annulus and at the other end to two &#34;one-way&#34; passageways (28, 29) linking it to the reference-gas-containing chamber (22) via a chamber-contained second piston (23).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to tools used in the testing of subterraneanwells, and concerns in particular the mechanism by which suchtools--especially but not exclusively those for use inhydrocarbon-bearing wells--are operated.

2. Description of The Prior Art

Whether at sea or on land, the first stages in the production of a newhydrocarbon well--an oil well--are the drilling of the well bore itselfthrough the various formations within the earth's crust beneath thedrilling rig, followed by "casing" (the introduction and cementing intoposition of piping which will serve to support and line the bore) andthe introduction into the bore, at the depth of a formation of interest,of a device known as a packer, into which inner tubing (of smallerdiameter than the casing) can subsequently be lodged.

The next work carried out is normally some programme of testing, for thepurpose of evaluating the production potential of the chosen formation.The testing procedure usually involves the measurement of downholetemperatures and pressures, in both static and flow conditions (thelatter being when fluid from the relevant formation is allowed to flowinto and up the well), and the subsequent calculation of various wellparameters. To collect the necessary data there is used a test string--alength of tubing containing the tools required for the testing--that islowered into the well bore to the required (test) depth. Either thepacker has previously been placed at that depth, and the test string isthen set into the packer, or the packer is sent down as part of the teststring, and then set into place in the bore; in any event, once thestring is set in the packer and the packer is set in the bore, thetubing of the string is isolated from the surrounding well.

One essential component of the test string is a valve known as thedownhole valve, which is used to control the flow of fluid out of theformation and into and up the well tubing. The density of drilling fluidin the tubing above this valve is adjusted such that its hydrostaticpressure at the depth of the formation is lower than the formation fluidpressure. Thus, when the valve is opened, formation fluid is permittedto enter the well bore through perforations in the casing and flow intothe tubing string (and possibly to the surface therethrough). Thiscontrasts with the situation during drilling, when the drilling mud mustexert a hydrostatic pressure greater than the formation fluid pressurein order to prevent the formation fluid's escape to the surface.

The operation of the various tools included in the downhole test string,including the opening and closing of the downhole valve itself--and,consequently, the control of the testing procedure--can be effectedusing one of three main types of mechanism. These types are thoseactuated by reciprocal motion of the pipe string (the inner tube, ofwhich the test string constitutes a part), by rotational motion of thepipe string, or by changes in the pressure differential between thetubing and the annular space which surrounds it in the well--hereinafterreferred to simply as "the annulus". Test strings wherein the toolsthereof are activated by changes in annulus pressure are at present muchin vogue, and it is this type of mechanism with which the invention isparticularly concerned.

A mechanism of the annulus pressure-responsive type requires theprovision and maintenance of a fixed "reference" pressure within thetool. This, used in conjunction with an adjustable (and higher) annuluspressure, allows the establishment of the chosen pressure differentialnecessary to control the operation of the appropriate component of thetest string.

To ensure that the downhole tools operate within a narrow known band ofapplied annulus pressure, it is essential that a constant referencepressure be established within the tool string. A convenient suchpressure to trap is the hydrostatic ambient (annulus) pressureexperienced by the string after it has been lowered down the well boreand set into the packer. This annulus pressure may, through a suitableconnection, be communicated to a gas-filled pressure chamber within thestring. However, once trapped the reference pressure must be isolatedfrom both the annulus and the tubing so that fluctuations in thepressures therein will not affect the reference pressure. Allowance mustalso be made for the commonly-encountered situation wherein there is apressure increase within the tubing, during stabbing into the packer,due to a "pistoning" effect (the annulus liquid being displaced by thedescending tubing can no longer escape up past the tubing once thelatter has reached, and is being stabbed into, the packer, so there is apressure build-up)--this excess pressure must be dissipated, and notcommunicated to the reference pressure chamber.

Variations in environmental temperature tend, via thermal expansion andcontraction of the pressurised gas, to alter the reference pressure, andso it is unfortunately also preferable to provide some means ofcompensating for this. Finally, additional temperature compensation maybe required if, as is quite common, certain procedures known in the Artas stimulation, which attempt to improve the oil yield of the formation,are employed once the initial well testing is completed Two examples ofsuch procedures are hydraulic fracturing and acid stimulation. Theirdetails are not relevant here, except inasmuch as they may require thepumping to the formation, via the test string, of fluids that are coldrelative to the formation temperature--acids, for example. A pumpingoperation of this kind will cause the reference pressure to drop, due tocontraction of the gas as it cools, unless some provision is made tomaintain it--and, furthermore, the pressure will rise again once thepumping has ceased unless once more it is adjusted. Analogous problemscan similarly occur during the pumping (albeit rare) of hot fluids tothe formation--for example, to help remove waxy deposits blocking theperforations in the casing.

All these situations, then, require some suitable means first ofisolating and then of maintaining the reference pressure in order thatit should remain constant (normally at the true hydrostatic pressure)under any foreseeable conditions, thus allowing a known pressuredifferential to be created between the tool and the annulus simply byraising the annulus pressure to a predetermined level.

It is these means that the invention seeks to provide. Firstly, theinvention proposes that reference pressure within the test string betrapped by a novel mechanism wherein a valve drivable into a closedposition by a first piston open to annulus pressure first defines, andthen defines and closes, the open-to-tubing-pressure entrance to apassageway leading to a reference-gas-containing chamber via a secondpiston therewithin. Using this mechanism, firstly, as the open-endedtest string is lowered into the wall bore, tubing pressure is inequilibrium with annulus pressure, and is communicated via thepassageway entrance and the chamber-contained piston to the referencegas, and secondly, after the test string has been stabbed into thepacker, so isolating tubing pressure from annulus pressure, a momentaryincrease in annulus pressure will cause the first piston to move todrive the valve into the passageway-closed position, thus effectivelysealing off the trapped reference gas from any further pressure changes.

Secondly, the invention proposes a new mechanism by which compensationcan be made for the effect of downhole temperature changes on the gas ina reference pressure chamber, in which mechanism there is ahydraulic-liquid-containing chamber which is connected at one end, via apiston thereat, to a vent to annulus and at the other end to two"one-way" passageways linking it to the reference-gas-containing chambervia a chamber-contained second piston. With this mechanism, upon cooling(and thus contraction and pressure reduction) of the reference gas theresultant excess annulus liquid pressure is communicated to, and exertedon, the second piston via the first piston and the hydraulic liquid,thus causing a movement of the second piston which will re-compress thegas and restore reference pressure. Similarly, upon heating (andexpansion and pressure increase) of the reference gas, the resultantexcess gas pressure is communicated to, and exerted upon, the firstpiston via the second piston and the hydraulic liquid, thus causing amovement of the first piston to vent chamber-contained annulus fluid,and thereby allowing movement of the second piston which will decompressthe gas and restore reference pressure.

BRIEF SUMMARY OF THE INVENTION

In one aspect, therefore, this invention provides a reference pressuretool containing therewithin a chamber holding a reference pressure gasand having means for trapping ambient pressure therein, which trappingmeans comprises:

a valve drivable into a closed position by a first piston open toannulus pressure; and

a passageway defined by the valve body, and closed by the valve when thelatter is in its closed position, which passageway has an entrance opento tubing pressure and leads to the reference-gas-containing chamber viaa chamber-contained second piston;

whereby tubing pressure is communicated to the reference gas, via thepassageway entrance and the chamber-contained piston, until an appliedincrease in annulus pressure over tubing pressure causes the firstpiston to move to drive the valve into the passageway-closed position,thus effectively sealing off the trapped reference gas from any furtherpressure changes.

DETAILED SUMMARY OF THE INVENTION

In a second aspect, therefore, this invention provides a referencepressure tool containing therewithin a chamber holding a referencepressure gas and having means for compensating for the effect oftemperature changes on the gas, which compensation means comprises:

a hydraulic-liquid-containing chamber connected at one end, via a pistonthereat, to a vent to annulus;

two passageways, each containing a one-way valve acting in the oppositedirection to that in the other, which passageways link the other end ofthe hydraulic-liquid-containing chamber to the reference-gas-containingchamber via a chamber-contained second piston;

whereby, upon thermally-induced pressure reduction of the reference gasthe resultant excess annulus liquid pressure is communicated via thefirst piston and the hydraulic liquid to the second piston, which thenmoves to re-compress the gas, whilst upon thermally-induced pressureincrease of the reference gas the resultant excess gas pressure iscommunicated via the second piston and the hydraulic liquid to the firstpiston such that the second piston moves initially to decompress the gaswhile the first piston moves to vent chamber-contained annulus liquid.

In its first aspect the invention provides a reference pressure toolincorporating means for trapping ambient tubing pressure within areference gas chamber therein. Although notionally the chamber might beof any shape, configuration and size, it is most conveniently an annularchamber constructed within the walls of the test tubing. These walls areabout 1 cm (0.5 in) thick; it is relatively easy to provide therewithinan annular chamber having a "cross sectional" thickness of around 1 cm(0.5 in). As to the size (volume) of the chamber, this naturally dependson the number of tools that the test string incorporates and that areoperated by pressurised liquid derived ultimately from the gas in thechamber. In general, however, it will be desirable to have at least 13liters (800 in³) of pressurised reference gas.

The reference pressure gas itself may be any gas that is both capable ofremaining gaseous under the downhole ambient conditions and non-toxicand non-corrosive. That gas commonly used is nitrogen. While this gasmay be introduced into the pressure chamber at normal pressures (that isto say, at 1 atmosphere), it is in fact much preferred to pump the gasin at a higher pressure--in the neighbourhood of 135 Bar (2000psi)--which ensures that the relevant floating piston(s) will havesufficient freedom of movement at the test string's planned operatingdepth.

The reference pressure tool of the invention allows ambient tubingpressure at the operating depth to be trapped and utilised thereafter asa reference pressure against which annulus pressure can be used toprovide an excess pressure to operate the various tools in the teststring. The trapping means comprises a piston-driven valve defining (andclosing) a passageway open to tubing pressure and leading via anotherpiston to the gas chamber.

In much the same way that the gas chamber can be of any form but ispreferably annular, being constructed within the tube walls, so theother major components of the trapping means are similarly preferablyannular, fitting within or adjacent the tube walls. Thus, the valve ismost conveniently a sleeve valve, internally mounted of the tubing andsliding along the tube from an initial open position to a final closedposition, and comprising a tubular valve body bearing a valve memberwhich is itself a ring seal that is moved along to and into contact withan internal tubing wall (defining the passageway, as discussed below).The first piston (which is conveniently a "floating" piston without acon-rod connecting it to any other part of the tool) is also mostconveniently annular. Moreover, although it would be possible to use apiston conventionally mounted between the oppssing side walls of achamber, it is in fact preferred to employ a step-form sleevepiston--that is to say, a piston in the form of a sliding sleeve halfwayalong the sliding face of which is a step effectively constituting thedriven face thereof (against which pressure is applied to drive thepiston), both the thicker and thinner sleeve portions above and belowthe step having ring seals that seal the piston to the surface againstwhich it slides. Such a stepped sliding-sleeve piston is shown in theaccompanying Drawings, and described hereinafter.

The piston can drive the valve in any convenient way. Advantageously,however, in effect it merely abuts one end of the valve body, and inoperation simply pushes the valve body from its "open" to its "closed"position.

The valve body, together with an internal surface of the tube, definespart--an annular part--of an internal passageway the rest of which maybe a narrow "pipe" formed within the tube walls. Along this passagewayin operation can flow annulus fluid contained within the tube--unless,of course the valve has moved to its "closed" position, in which casethe passageway is sealed shut by the valve member itself. Thispassageway is open at one end to the inside of the tube, and thus totubing pressure, and the necessary opening is conveniently at the"annular" portion end--and, indeed, by way of an aperture in and throughthe valve body. At the other end (the "pipe" end) the passageway opensinto the reference pressure gas chamber, but a direct connection betweenthe passageway and the gas in the chamber is prevented by a piston--inthe preferred case, a floating annular piston--operatively mountedwithin the gas chamber at or adjacent the passageway's opening thereto.

In a preferred embodiment of the invention there is within thepassageway a non-return valve preventing the flow ofpassageway-contained tubing liquid back towards (and possibly out of)the end of the passageway open to tubing pressure. This prevents loss ofreference pressure immediately after stabbing-in should the formationpressure be less than annulus pressure (as may sometimes be the case).The non-return valve may take any convenient form, but preferably it isannular, mounted within an annular valve chamber forming a widened partof the annular portion of the passageway to the gas chamber, andspring-loaded into a position where it closes off the egress of theupstream section of the passageway into the valve chamber.

In operation, the open-ended test string containing the referencepressure tool is lowered slowly into the well bore, and as this occurstubing pressure is communicated to the reference gas via the passagewayentrance and the chamber-contained piston, whereupon drilling liquid(tubing and annulus) pressure will act both upon the first,valve-driving piston and upon the second, gas-chamber-contained piston(in the latter case, via the passageway opening from the tubing).However, the tool is not affected in any way until it has been loweredbeyond the depth at which the downhole hydrostatic pressure exerted bythe drilling liquid exceeds the pressure of the pre-pressurizedreference gas within the chamber. Upon passing this depth, the excessliquid pressure subsequently exerted on the reference gas via thechamber-contained piston progressively compresses the reference gas sothat the pressure thereof is always equal to the ambient hydrostaticpressure. This compression process continues until the required testdepth is reached, whereupon the test string is "stabbed in" to thepacker--that is to say, it is sealingly lodged therein--thus isolating,for the first time, the tubing of the tool from the annulus.

Following stabbing-in, the required reference pressure contained withinthe gas chamber must be trapped by driving the valve into its closedposition. This is achieved by momentarily increasing annulus pressureover tubing pressure. This new increased pressure--applied to theannulus from the head of the well in any convenient way--creates apressure differential across the valve-driving piston, which nowexperiences hydrostatic (tubing) pressure on one side and the applied(and higher) annulus pressure on the other. The piston therefore moves,and as it does so drives the valve into its closed position, thussealing the passageway leading to the reference gas chamber, and soeffectively isolating the gas therein from any further pressure changes.

As the test string is slowly lowered down the well bore as justdescribed the pressures of the drilling liquid within tubing and annulusare continuously equalised by the unrestricted flow of that liquidaround the test string. It will, however, be appreciated that duringstabbing-in there is no longer any chance for a flow of displaceddrilling liquid up past the tube to equalise these pressures completely.There results a "piston effect", which causes tubing pressure toincrease over annulus pressure; if uncompensated, this will result inthe subsequently-trapped reference pressure being too high, due tocapture of the (excess) tubing pressure instead of the desiredhydrostatic pressure. Accordingly, in a preferred form the referencepressure tool of the invention incorporates a mechanism by which theexcess tubing pressure generated on stabbing-in can be bled off toannulus without being communicated to the reference gas chamber. Thatmechanism conveniently employs a one-way bleed valve opening to annulusand positioned along the passageway to the reference gas chamber, whichbleed valve opens whenever tubing pressure markedly exceeds annuluspressure by some pre-set value. In a tool which incorporates such amechanism in addition to the preferred non-return valve describedhereinbefore, the relative positioning of the two valves along thepassageway may be such that the bleed valve is either upstream ordownstream of the non-return valve, though having regard to the limitedspace available the valve is very preferably an annular valve (like thenon-return valve) situated upstream. Thus the bleed valve is preferablyco-axial with the non-return valve's chamber, and operatively connectedbetween the latter chamber and a port to annulus, spring-loaded into aposition where it blocks the egress of the connection to the latterchamber, and so prevents ingress of liquid thereinto.

In its second aspect the invention also provides a reference pressuretool incorporating a gas-filled reference pressure chamber. The remarkscontained hereinbefore regarding the nature of both chamber and gas inthe first tool are equally applicable in this case, and accordingly nofurther comment will be made here--save, perhaps, to point out that thesecond tool may naturally be one of the first tool's type as describedherein.

This second reference pressure tool includes means for compensating forthe effect of temperature changes on the gas--specifically, meansutilising a chamber of hydraulic liquid connected at one end (via apiston thereat) to a port to annulus, and at the other to another pistonin the reference gas chamber via two "one-way" passageways. The liquidchamber is conveniently annular, and constructed within the tube wallsin much the same way as the reference gas chamber. Its dimensions, andhence the volume of fluid contained therewithin, depend at least in parton the magnitude of the temperature range that is anticipated.Generally, however, a volume of 13 liters (800 in³) will be sufficient.

The hydraulic liquid requires no special properties save those ofremaining liquid in all foreseeable circumstances, and of beinggenerally inert-- non-toxic, non-corrosive, and, especially,non-explosive. Suitable liquids are silicone oils, as is well known inthe Art.

The piston separating the liquid chamber from the port to annulus is, ina preferred embodiment of the invention, another annular, floatingpiston.

The liquid chamber is linked at its other end (the end not connected tothe port to annulus) to two passageways leading to a piston within thereference gas chamber. In a reference pressure tool incorporating boththe reference pressure trapping means of the invention and thetemperature compensation means presently being described, it may beappreciated that the gas chamber will thus be bounded by two pistons(conveniently both of the floating annular kind), one of which isadjacent the open-to-tubing passageway required for the trapping ofreference pressure, and the other of which links (indirectly) the gaschamber to the hydraulic liquid chamber.

The passageways linking the gas- and hydraulic-liquid-chambers areconveniently housed within the tube walls, and of narrow tubular form.Each passageway has within its length a one-way valve, very preferablyof a pressure-sensitive variety. Not only does this valve permit onlyunidirectional flow therethrough (and the arrangement is such that onepassageway allows flow only in one direction whilst the other allowsflow only in the other direction), but in addition the flow isrestricted to an extremely low rate (about 1 cc per 10 minutes)regardless of the pressure drop across the valve (the reason for this isdiscussed hereinafter in more detail with reference to the Drawings, butbriefly it is to prevent sudden annulus pressure changes which affectthe pressure of the hydraulic liquid from further affecting the pressureof the gas in the reference pressure chamber connected thereto). Thus,provided the pressure differential is low enough, in one passagewayhydraulic liquid may flow from the chamber up to the piston only, whilstin the other the reverse is true. Valves of this one-way, restrictornature are well known, and commercially available.

In operation, as the test string is lowered into the well bore thehydrostatic pressure will at some point exceed the pressure of thechamber-contained hydraulic liquid. When this happens, drilling liquidfrom the annulus will enter the port, and will cause the pistoncontained within the hydraulic liquid chamber to "move" to pressurizethe liquid, thus continuously adjusting the pressure thereof to thehydrostatic pressure. The same pressure will also be communicated to theliquid contained within the passageway permitting flow to the gaschamber (the liquid in the other passageway will remain at its initialvalue, since the required direction of flow to increase it is preventedby the one-way valve).

Following stabbing-in and the trapping of the reference pressure, anyreduction in the ambient temperature--such as might occur during astimulation with cold acid--will in the first instance cause thepressure of the gas within the reference pressure chamber to drop(initially the volume of the gas notionally stays the same--it is thatvolume contained within the piston-bounded chamber). If the referencepressure were to remain at this reduced level problems would arise inoperating the test string because the application to the annulus liquidof a pressure a specific amount higher than the expected referencepressure (in order to create the pressure differential by which one ofthe tools is activated) would no longer necessarily have the desiredeffect when measured against the now reduced reference pressure.However, in the tool of the invention a (thermally-induced) pressuredrop of this nature gives rise to a pressure differential across thegas-chamber-contained piston of the temperature compensation means. Onone side, this piston experiences the reduced gas pressure, and on theother it experiences the unchanged (and therefore higher)hydrostatic--that is, annulus--pressure which is being communicated toit via the hydraulic-liquid-filled passageway and chamber and theopen-to-annulus piston. The gas-chamber piston therefore moves under theinfluence of the excess liquid pressure in such a way that the volume ofthe reference gas chamber bounded thereby is decreased. The pressure ofthe gas within the chamber thus increases until it once more equals theoriginal hydrostatic (reference) pressure. In this way the correctoperation of the test string in response to applied annulus pressure isensured even during a drop in ambient downhole temperature.

The described temperature reduction may eventually be reversed (as when,for example, acid stimulation ceases, and the ambient temperatureincreases to the normal, "background" level), and when this happens theresulting increase in reference gas pressure (as the gas heats up) mustsuitably be allowed for. In the mechanism of the invention there willnow be a pressure differential across that piston between the gaschamber and the liquid chamber such that the higher pressure is thatexerted by the reference gas. The piston thus moves to allow the gas toexpand (thereby reducing its pressure). As it does so, the hydraulicliquid is pushed through the passageway and liquid chamber, and in turndrives the open-to-annulus piston to vent annulus liquid from thetool--a process that continues until reference pressure has beenrestored to the desired value.

Provided it is not too large, any temperature variation--and, indeed,any sequence of such variations--occurring down the well can be suitablycompensated by adjustments of the types just described, thereby ensuringthat the pressure differential required for test tool operation mayalways correctly be achieved by application of a previously-calculatedannulus pressure.

The materials employed in the construction of the various components ofthe two inventions hereinbefore described may be any of those normallyutilised in the Art for similar construction. Thus, for example, thetubing of the tool may be of a low carbon alloy steel, and the valvegear may be of any suitably non-corrodible substance (for example,INCONEL).

Although this invention has been described in the main with reference tooil wells, it can in fact be of use in any kind of well--oil, gas orwater, for instance--where it is necessary or desirable to investigatethe downhole formations.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention is now described, though by way ofillustration only, with reference to the accompanying diagrammaticdrawings in which:

FIG. 1 is a simplified cross sectional view of an offshore oil well witha test string including a tool of the invention;

FIGS. 2A/B show a tool of the invention as it appears in cross-sectionprior to stabbing into the packer;

FIGS. 3A/B show the tool of FIG. 2 after stabbing into the packer andapplying a high annulus pressure;

FIG. 4 shows the B section of the tool of FIG. 2 after a drop in ambientdownhole temperature; and

FIG. 5 shows the B section of the tool of FIG. 2 after an increase inambient downhole temperature.

In each of FIGS. 2 and 3 the A and B sections are, in reality,connected--the left side of the B figure runs on from the right side ofthe A figure.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a floating drilling rig (101, not shown in detail) fromwhich has been drilled an oil well (generally 102) having a well bore(103) reaching down to a rock stratum constituting the formation (109)of interest. Located at the top of the well bore 103 is a blow-outpreventer mechanism (BOP; 104, not shown in detail) which is connectedto the rig 101 by a marine riser (105). Cemented into the well bore 103are a shallow casing (106) and a deep casing (107); the lower end of thelatter has a multitude of perforations (as 108) permitting communicationbetween the well bore 103 and the oil formation 109.

Situated within the well bore 103 is a test string (110) comprisingtubing (113) ending in a set of test tools (see below). The string 110is set at its lower end into a packer (111), and a seal sleeve (112)seals the packer 111 to the test string 110, thus isolating the tubing113 thereof from the annulus (114).

Above the seal sleeve 112 is a gauge carrier (115) which containselectronic or mechanical gauges (not shown) which collect downholepressure and temperature data during the test sequence. Above the gaugecarrier 115 are the constant pressure reference tool (117) and thedownhole valve (118; the operation of which enables the test sequence tobe carried out). A circulating sleeve (119) permits removal of anyformation fluid remaining within the test string 110 prior to itswithdrawal from the well bore 103. At the top of the test string is asubsea test tree (120) which serves both as a primary safety valve andas a support for the rest of the test string 110.

FIGS. 2 to 5 show a constant pressure reference tool 117 of theinvention having a main housing (1) and the tubing internal bore (2). Atthe lower end (at the left as shown) of the tool there is within anannular chamber (10) a floating annular stepped sliding sleeve piston(3; shown hatched) which communicates with liquid (not shown) in theannulus (not shown specifically--it is the volume "outside" thehousing 1) by way of a port (5) to annulus (the annulus liquid isapplied to the face of a step halfway along the sleeve, and pressesthereagainst so as in operation to drive the piston towards the right asshown). Communication between annulus and tubing 2 around piston 3 isprevented by elastomer seals (32, 34).

The floating piston 3 is in direct driving contact with a sliding (seal)sleeve valve (4; shown hatched) having elastomer seals (12) and which,when driven by the piston 3, is capable of movement (to the right asshown) along the annular chamber 10. A port (6) through the sleeve 4permits communication between tubing 2 and annular chamber 10. Since,prior to stabbing in, the tubing 2 is open to annulus, the liquidpressures acting on each side of floating piston 3 through ports 5 and 6are equal, and so no movement of piston 3 (or sleeve 4) occurs.

A narrow annular passageway (30) leads from the annular chamber 10 to aone-way spring-loaded valve (13) which permits liquid flow therethroughonce the force of its valve spring (15) has been overcome, but whichprevents the return of this liquid. Beyond valve 13 are another,pipe-like, passageway (19) and a further one-way spring-loaded valve(14) with an associated spring (16). The valve 14 will only allow liquidto pass through it if the pressure thereof markedly exceeds the pressureof the liquid in the annulus. Downstream of the valve 14 is a port (7)to annulus.

Passageway 19 leads to an annular, reference-gas-containing referencepressure chamber (22; the gas is usually nitrogen), confined at eitherend by a floating piston (20, 23). A port (37) permits directcommunication between gas chamber 22 and outside the tubing and the gasmay be charged into the chamber 22 therethrough (after which the port issealed up). On the other side of the piston 23 there opens a pair ofnarrow passageways (26a and 26b; not shown separately in the Drawings)which lead, via pressure-sensitive, one-way valves (28, 29 respectively;not shown in detail) to an annular chamber (27) containing hydraulicliquid. These two valves 28, 29 are pressure-sensitive in that theyremain open while the pressure across them stays below a certain,predetermined, threshold value, but close immediately that thresholdvalue is reached or exceeded. The reason for this is so that when, as isdiscussed hereinafter, there is a sudden and substantial rise (or fall)in annulus pressure, the relevant valve will close to prevent transferof this pressure change on into the rest of the system, but that such apressure transfer will be permitted if the change in annulus pressure issmall or slow. The liquid chamber 27 is connected to a port (24) toannulus via a further floating piston (25). Valve 28 permits liquid flowalong passageway 26a from chamber 27 towards piston 23 only, whereasvalve 29 allows liquid flow away from piston 23 only.

Before the tool is lowered, as part of the test string, into the wellbore, the gas within the reference pressure chamber 22 and the hydraulicliquid within chamber 27 are both adjusted to a pressure of 135 Bar(2000 psi). During the lowering process, liquid in the annulus andtubing 2 surrounds the tool, enters the ports 5, 6, 7 and 24, and fillsannular chamber 10 and passageway 19 (the liquid does not, however, passvalve 14 since the liquid pressures either side thereof--in tubing 2 andthe annulus via port 7--are equal).

The liquid does not at first enter the reference pressure chamber 22 orthe hydraulic liquid chamber 27 because these have initial internalpressures greater than the hydrostatic pressure exerted by the wellliquid. When the tool reaches a certain depth, however, hydrostaticpressure will exceed the pressure of the reference gas and of thehydraulic liquid. This hydrostatic pressure will act upon the gas,having been communicated through port 6 to chamber 10 and alongpassageway 19 to piston 20. This piston will thus move along chamber 22,to pressurize the gas therein until pressure balance is restored (whenthe gas reaches hydrostatic pressure). Similarly, well liquid enteringport 24 will push piston 25 into the liquid chamber 27 until thepressures within the chamber and passageway 26a equal the instantaneoushydrostatic pressure (the pressure of the liquid within passageway 26bremains at its initial value due to the action of valve 29).

When, having reached the required test depth, the test string is stabbedinto the packer, the pressure within the tubing 2 will tend to increaseabove the hydrostatic pressure as a result of a "pistoning" effect. Whenthis happens, valve 14 opens and excess liquid from within the tool isvented to the annulus via port 7 until tubing and hydrostatic pressuresare again equal. The pressure of the gas within annular chamber 22 thusremains at the hydrostatic pressure--and indeed non-return valve 13ensures that it does remain so even if, because of a low formationpressure, tubing pressure should drop below annulus hydrostaticpressure.

After the test string has been stabbed into the packer, the tubing 2 andthe annulus are isolated from each other. It is then necessary suitablyto isolate the reference pressure trapped within chamber 22. To achievethis, the annulus pressure is briefly increased (by a suitable forceapplied at the surface). This increased annulus pressure is observed atports 5, 7 and 24, but not at port 6 (which still experiencestubing--hydrostatic--pressure only), so now there is a pressuredifferential across floating piston 3. This differential forces thepiston, together with seal sleeve 4, along annular chamber 10, bringingthe sleeve into its "closed" position (as shown in FIG. 3), where port 6is closed and the passageway 30 is sealed off by elastomer seal 12. Theincreased annulus pressure experienced at port 7 cannot influencepressure within the tool because of the presence of one-way valve 14. Atport 24, however, the increased annulus pressure will cause movement ofpiston 25 such that the hydraulic liquid within chamber 27 ispressurized until it also attains this increased pressure. However,since the pressure increase in the annulus is effected suddenly, itproduces a large pressure differential--greater than the pre-setvalue--across restrictor valve 28, which accordingly closes, and thusprevents the increased annulus pressure from being transmitted to thereference gas.

Once the applied annulus pressure has caused the required movement ofpiston 3 and sleeve valve 4, the excess pressure is bled off at surfaceso that annulus hydrostatic pressure is once more the true ambientpressure. This procedure is accompanied by the venting of tool-containedannulus liquid from port 24 by piston 25 until the hydraulic liquidwithin chamber 27 also returns to hydrostatic pressure.

FIGS. 4 and 5 show the effect on the tool of changes in downholetemperature.

FIG. 4 shows the effect of a drop in downhole temperature. Any resultant(small) drop in the pressure of the hydraulic liquid within chamber 27is rectified by movement of piston 25 initiated by the correspondingexcess hydrostatic pressure exerted thereon by annulus liquid. Thereference is, however, susceptible to a much more significant pressuredrop. This results in pressure differentials arising across both of thegas-chamber-contained pistons 20 and 23 which drive these pistonstowards each other, re-pressurizing the gas. Piston 20 will move onlyslightly (there is only a small volume of liquid behind it, and hencepressure balance thereacross is soon restored), but piston 23 will moveas far as is necessary to re-establish the original reference pressurein the gas (the hydraulic liquid in passageway 26 and chamber 27 isalways maintained at hydrostatic pressure by influx of annulus liquid atport 24 as just described).

The effect of a rise in the ambient downhole temperature is shown inFIGS. 4 and 5. The reference gas pressure (and, much less significantly,that of the hydraulic liquid) also rises. The hydraulic liquid pressureis maintained by flow of annulus liquid through port 24. In the case ofthe gas, pressure differentials are created across floating pistons 20and 23 which would tend to drive these pistons away from each other, toallow the reference pressure to adjust to the desired hydrostaticpressure. However, when floating piston 23 reaches the upper end of thegas chamber 22 it is unable to move further to reduce the pressuredifferential across it. Restoration of the reference pressure to itsoriginal value must therefore be effected by movement of piston 20. Asthis happens, the well liquid contained in the chamber 22 on the otherside of the piston 20, and in passageway 19, is pressurized. When itspressure exceeds hydrostatic pressure, valve 14 will open and ventexcess liquid to the annulus via port 7 until equilibrium is reached.

We claim:
 1. A reference pressure tool for use with a well test stringin the form of one or more length of pipe tubing having walls definingan internal bore such that in operation the tubing is inserted into aborehole of a well to be tested, and is subject to tubing pressurewithin the tubing bore and to annulus pressure outside the tubing, thereference pressure tool comprising within its tubing a chamber holding areference pressure gas and having trapping means for trapping ambientpressure therein, said trapping means including a first piston actedupon by annulus pressure, and a valve having a valve body moveablebetween a valve-open position and a valve-closed position, and drivableinto the valve-closed position by said first piston;said chamber alsocontaining a second piston, and said trapping means also including apassageway defined by the valve body, said passageway being closed bythe valve when the valve body is in its closed position, said passagewayhaving within the valve body an entrance open to tubing pressure, andsaid passageway leading to said reference gas containing chamber viasaid chamber-contained second piston; whereby tubing pressure iscommunicated to the reference gas, via said passageway entrance and thechamber-contained second piston, until an applied increase in annuluspressure over tubing pressure causes said first piston to move to drivesaid valve body into the passageway-closed position, thus effectivelysealing off the trapped reference gas from any further pressure changes.2. A tool as claimed in claim 1, wherein the chamber holding thereference pressure gas is an annular chamber constructed within thewalls of the test string tubing.
 3. A tool as claimed in claim 1,wherein, to drive the valve, the first piston merely abuts one end ofthe valve body, and in operation simply pushes the valve body from itsopen to its closed position.
 4. A tool as claimed in claim 1, whereinthere is within the passageway a non-return valve preventing the flow ofpassageway-contained tubing liquid back towards, and possibly out of,the end of the passageway open to tubing pressure.
 5. A tool as claimedin claim 4, wherein the non-return valve is annular, mounted within anannular valve chamber forming a widened part of the annular portion ofthe passageway to the gas chamber, and spring-loaded into a positionwhere it closes off the egress of the upstream section of the passagewayinto the valve chamber.
 6. A tool as claimed in claim 1, wherein thereis incorporated a mechanism by which the excess tubing pressuregenerated on stabbing-in can be bled off to annulus without beingcommunicated to the reference gas chamber.
 7. A tool as claimed in claim6, wherein the bleed-off mechanism employs a one-way bleed valve openingto annulus and positioned along the passageway to the reference gaschamber, which bleed valve opens whenever the pressure-trapping valve isopen and tubing pressure markedly exceeds annulus pressure by somepre-set value.
 8. A tool as claimed in claim 7, wherein the bleed valveis annular and co-axial with the non-return valve's chamber, andoperatively connected between the latter chamber and a port to annulus,spring-loaded into a position where it blocks the egress of theconnection to the latter chamber, and so prevents ingress of liquidthereinto.
 9. A well test string which incorporates a reference pressuretool as claimed in claim
 1. 10. A tool as claimed in claim 1, whereinthe valve body, together with an internal surface of the tube, definesan annular part of an internal passageway the rest of which is a narrowpipe formed within the tube walls, and wherein the passageway is open tothe inside of the tube, at the annular portion end, by way of anaperture in and through the valve body, while at the other end, the pipeend, the passageway opens into the reference pressure gas chamber via afloating annular piston operatively mounted within the gas chamber at oradjacent the passageway's opening thereto.
 11. A reference pressure toolfor use with a well test string in the form of one or more length ofpipe tubing having walls defining an internal bore such that inoperation the tubing is inserted into a borehole of a well to be tested,and is subject to tubing pressure within the tubing bore and to annuluspressure outside the tubing, the reference pressure tool comprising, anannular chamber constructed within the walls of the test string tubingand holding a reference pressure gas and having trapping means fortrapping ambient pressure therein, said trapping means including a firstpiston acted upon by annulus pressure, and a sleeve valve having atubular valve body mounted internally of the tubing and slidablymoveable along the tubing between an initial valve-open position and afinal valve-closed position, and driveable into the valve-closedposition by said first piston;said chamber also containing a secondpiston, and said trapping means also including a passageway defined bythe valve body, said passageway being closed by the sleeve valve whenthe valve body is in its closed position, said passageway having withinthe valve body an entrance open to tubing pressure, and said passagewayleading to said reference gas containing chamber via saidchamber-contained second piston; said tubular valve body bearing a valvemember which is itself a ring seal that is moved along to and intocontact with an internal tubing wall defining the passageway; wherebytubing pressure is communicated to the reference gas, via saidpassageway entrance and the chamber-contained second piston, until anapplied increase in annulus pressure over tubing pressure causes saidfirst piston to move to drive said valve body into the passageway-closedposition, thus effectively sealing off the trapped reference gas fromany further pressure changes.
 12. A tool as claimed in claim 11, whereinthe first piston is a floating piston, and is also annular, and is astep-form sleeve piston.
 13. A reference pressure tool for use with awell test string in the form of one or more length of pipe tubing havingwalls defining an internal bore such that in operation the tubing isinserted into a borehole of a well to be tested, and is subject totubing pressure within the tubing bore and to annulus pressure outsidethe tubing, the reference pressure tool containing within its tubing achamber holding a reference pressure gas and having means forcompensating for the effect of temperature changes on the gas, in whichtool the compensation means comprises,a compensation chamber forcontaining a hydraulic liquid, said chamber having two ends andcontaining spaced therebetween two pistons, and having at one end a ventto annulus to which the chamber contents are connected via one of saidtwo pistons; and two passageways, each containing a one-way valve actingin the opposite direction to that in the other, which passageways linkthe other end of said compensation chamber to the reference gascontaining chamber via the other piston of said two pistons; whereby,upon thermally-induced pressure reduction of the reference gas theresultant excess annulus liquid pressure is communicated via said onepiston and the hydraulic liquid to said other piston, which then movesto re-compress the gas, whilst upon thermally-induced pressure increaseof the reference gas the resultant excess gas pressure is communicatedvia said other piston and the hydraulic liquid to said one piston suchthat said other piston moves initially to decompress the gas while saidone piston moves to vent chamber-contained annulus liquid.
 14. A tool asclaimed in claim 13, wherein the liquid chamber is annular, andconstructed within the tube walls.
 15. A tool as claimed in claim 13,wherein the piston separating the liquid chamber from the port toannulus is an annular, floating piston.
 16. A tool as claimed in claim13, wherein the passageways linking the gas- andhydraulic-liquid-chambers are housed within the tube walls, and ofnarrow tubular form, and each has within its length a pressure-sensitiveone-way valve that restricts the flow therethrough to an extremely lowrate regardless of the pressure drop across the valve.
 17. A well teststring which incorporates a reference pressure tool as claimed in claim13.